1. Field of the Invention
The invention relates in general to an inverter, and more particularly to an inverter capable of switching working frequency.
2. Description of the Related Art
With the coming of the digital age and the popularization of the computer network, the display has been inevitably flattened and thinned. The inverter, which is a key component for activating a lamp in the flat panel display, determines the reliability and the stability of the lamp, and the performance of the inverter directly influences the display quality of the flat panel display. Due to the requirement in the market, the size of the display panel is getting larger and larger, and the length of the lamp is getting longer and longer. Thus, the working frequency of the lamp has a greater influence on the activation process. So, how to select a suitable working frequency has become a great focus in the industry.
FIG. 1 is a block diagram showing a conventional inverter. Referring to FIG. 1, a conventional inverter 100 includes a power device 112, a transformer 114, a feedback circuit 130 and a control unit 140. The power device 112 receives a DC input voltage from a DC power 500, converts the DC input voltage into a first AC voltage according to a control signal, and then outputs the first AC voltage. The transformer 114 receives the first AC voltage, transforms the first AC voltage into a second AC voltage, and then outputs the second AC voltage to a lamp 600. The lamp 600 is activated by the second AC voltage and generates a feedback current. The feedback circuit 130 receives the feedback current and outputs a feedback voltage according to the feedback current. The control unit 140 includes a resonance circuit 142 and a control device 144. The resonance circuit 142 includes a resonance capacitor Ct and a resonance resistor Rt. The control unit 140 can generate a working frequency according to the resonance resistor Rt and the resonance capacitor Ct. The control device 144 receives the feedback voltage and generates the control signal for controlling the AC voltage outputted from the power device 112 to the lamp 600 according to the working frequency and the feedback voltage.
The lamp 600 requires different working frequencies in different working states. For example, at a transient period after the inverter 100 initially activates the lamp 600, the gas and the metal elements in the lamp 600 are activated but not stable. This problem becomes more serious especially in a low-temperature environment. In this case, a lower working frequency, such as the working frequency of 45 KHZ, is required to activate the lamp 600 such that the lamp 600 has a better performance. After the transient period, the lamp 600 reaches a steady state, in which the gas and the metal elements in the lamp 600 have been activated to reach a stable state. In this case, a working frequency higher than 45 KHZ is required to activate the lamp 600. For example, the working frequency of 50 KHZ is required to activate the lamp 600 such that the lamp 600 has a better performance.
However, the conventional inverter 100 can only activate the lamp 600 with the same working frequency. The designer for the conventional inverter 100 can only select either the frequency suitable for the initial activation of the lamp or the frequency suitable for the steady state of the lamp as the final working frequency. If the working frequency suitable for the steady state is selected, the lamp 600 tends to flicker due to the impedance matching problem in the aforementioned initial transient period. Thus, the waveform of the AC voltage is unstable during that transient period. If the working frequency suitable for aforementioned initial transient period is selected, the lamp 600 can work normally in a predetermined period after the lamp 600 is initially activated. However, the problems of unsymmetry of the waveforms of the positive half cycle and the negative half cycle of the AC voltage, the waveform distortion, and the uneven currents among several lamps would occur after that transient period. Thus, the problem of the insufficient luminance, the nonuniform luminance or the shortened lifetime will further be caused. The problem will become more serious especially when the dimension of the lamp 600 is getting larger and larger.